From all my dealings with them, they don't believe that 'baptism in the Spirit' is a perfecting event either. It doesn't seem to have the remotest resemblance to the doctrine of 'entire sanctification' - to me. (I believe the indwelling/filling with the Spirit occurs at the moment of salvation, so that in effect puts me in a second-class Christian caste according to the 'baptism of the Spirit' folks...)
Who exactly are 'them'? Pentecostals and people involved in "Holiness Movement" churches--both of which have their roots in deviations of Methodist doctrine--but particularly Holiness people, believe that "the baptism in the Spirit" is entire sanctification. The Pentecostal has two 'camps': those who say that BitS (baptism in the Spirit) is a work of empowerment but not necessarily of holiness, and those who say that it is both empowerment AND holiness.
In either case, they believe that it is a 'second work of grace.' While orthodox Methodism (which has been in short supply since at LEAST the late 1800s) agrees that entire sanctification is a second work of grace--they equate it to the completion of the work started in regeneration, though it itself is also not ever truly 'complete' in a sense that doesn't mean it can still grow--they say that Baptism in the Spirit occurs at the moment of salvation, for "if anyone have not the Spirit of Christ, he is none of His."